1. Field of the Invention
The present invention relates to fabrication of semiconductor devices, and more particularly, to a method for fabricating semiconductor memory devices.
2. Description of Prior Art
Memory devices have become indispensable and also the most widely used device in computer-related systems. In the current information age, memory devices are compact in package with high storage capacity and processing speed. A non-volatile memory device is one that retains its data even when it is not supplied with power. This characteristic allows the non-volatile memory device to serve as a fixed data storage device that permanently stores repeatedly used programs and/or tables of data.
Conventional methods for fabricating non-volatile memory devices involve many steps which are complicated and time-consuming to perform. In general, most non-volatile memory devices are similar in structure except for the specific data to be permanently stored therein. Accordingly, a semiconductor manufacturer usually fabricates the nonvolatile memory device to a semi-finished product without performing the factory-programming step to store the permanent data. The semi-finished product is then stocked in a warehouse waiting for a customer order. After a customer-specified programming is received, the factory-programming step is then performed. This allows the non-volatile memory device product to be delivered to the customer in a short time. Alternatively, some sophisticated customers prefer to program the semi-finished product themselves. Thus, the un-programmed semi-finished product is delivered to the customer.
FIG. 1 (Prior Art) is a top view of a conventional non-volatile semiconductor memory device, which is a mask-programmed read-only memory (ROM) device having buried bit lines. This ROM device includes lengthwise interleaved source/drain areas 10, 11, 12, 13 (referred to as "buried bit line") and crosswise interleaved polysilicon gate areas 20, 21, 22, 23 (referred to as "word line"). Suppose that the data readout operation is to be performed on a memory cell 55, a high voltage is fed to the word line 21 and to bit line 11. A low voltage is fed to the bit line 12. By conventional methods the word lines have high resistance and an RC delay will be high if the distance between the memory cell 55 and the voltage source is too long. This delay will cause the data readout operation to be slow and overall system efficiency is affected.
There is a need to enhance the performance of the non-volatile memory devices. Various approaches have been tested, but none of them provide a perfect solution to the high-resistance problem that causes signal delays in the word lines. Another approach is to use a substrate of different material such as GaAs which allows the transistors thus fabricated to have higher switching speed than those fabricated based on a silicon substrate, whereby the data readout speed from the ROM device is enhanced. However, in this approach the aforementioned high-resistance problem which causes signal delay on the word lines still exists. The signal delay is a result of RC delay due to high resistance of the word lines and the capacitance of the insulators and which is existent regardless of which material is used as the substrate.